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data-v-aa0e2252><a class="outline-link" href="#_3-null-判断运算符" data-v-aa0e2252>3.Null 判断运算符</a><!----></li><li style="" data-v-aa0e2252><a class="outline-link" href="#_4-逻辑赋值运算符" data-v-aa0e2252>4.逻辑赋值运算符</a><!----></li><!--]--></ul></nav></div><!--[--><!--[--><!--[--><div><a class="sponsors-aside-text" href="/">联系客服</a><img src="/images/qrcode.png"></div><!--]--><!--]--><!--]--><div class="VPCarbonAds" data-v-4fe9b7bd><img src="/logo.svg" alt="" srcset=""></div><!--[--><!--[--><!--[--><h1 class="tagline" data-v-3916b2f0><span class="accent" data-v-3916b2f0>Bin</span><br data-v-3916b2f0>QQ - 1974892005 </h1><!--]--><!--]--><!--]--></div></div><div class="content" data-v-4fe9b7bd><!--[--><!--]--><main data-v-4fe9b7bd><div style="position:relative;" class="vt-doc es6" data-v-4fe9b7bd><div><h1 id="_08【运算符的扩展】" tabindex="-1">08【运算符的扩展】 <a class="header-anchor" href="#_08【运算符的扩展】" aria-hidden="true">#</a></h1><h2 id="_1-指数运算符" tabindex="-1">1.指数运算符 <a class="header-anchor" href="#_1-指数运算符" aria-hidden="true">#</a></h2><p>ES2016 新增了一个指数运算符（<code>**</code>）。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">**</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// 4</span></span>
<span class="line"><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">**</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">3</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// 8</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>这个运算符的一个特点是右结合，而不是常见的左结合。多个指数运算符连用时，是从最右边开始计算的。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 相当于 2 ** (3 ** 2)</span></span>
<span class="line"><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">**</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">3</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">**</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 512</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，首先计算的是第二个指数运算符，而不是第一个。</p><p>指数运算符可以与等号结合，形成一个新的赋值运算符（<code>**=</code>）。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> a </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1.5</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">**=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于 a = a * a;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> b </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">4</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">b </span><span style="color:#89DDFF;">**=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">3</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于 b = b * b * b;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h2 id="_2-链判断运算符" tabindex="-1">2.链判断运算符 <a class="header-anchor" href="#_2-链判断运算符" aria-hidden="true">#</a></h2><p>编程实务中，如果读取对象内部的某个属性，往往需要判断一下，属性的上层对象是否存在。比如，读取<code>message.body.user.firstName</code>这个属性，安全的写法是写成下面这样。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 错误的写法</span></span>
<span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;">  firstName </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> message</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">body</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">user</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">firstName </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">default</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 正确的写法</span></span>
<span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> firstName </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> (message</span></span>
<span class="line"><span style="color:#A6ACCD;">  </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> message</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">body</span></span>
<span class="line"><span style="color:#A6ACCD;">  </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> message</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">body</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">user</span></span>
<span class="line"><span style="color:#A6ACCD;">  </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> message</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">body</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">user</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">firstName) </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">default</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面例子中，<code>firstName</code>属性在对象的第四层，所以需要判断四次，每一层是否有值。</p><p>三元运算符<code>?:</code>也常用于判断对象是否存在。</p><div class="language-"><pre><code>const fooInput = myForm.querySelector(&#39;input[name=foo]&#39;)
const fooValue = fooInput ? fooInput.value : undefined
</code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面例子中，必须先判断<code>fooInput</code>是否存在，才能读取<code>fooInput.value</code>。</p><p>这样的层层判断非常麻烦，因此 ES2020 引入了“链判断运算符”（optional chaining operator）<code>?.</code>，简化上面的写法。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> firstName </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> message</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">body</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">user</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">firstName </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">default</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> fooValue </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> myForm</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">querySelector</span><span style="color:#A6ACCD;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">input[name=foo]</span><span style="color:#89DDFF;">&#39;</span><span style="color:#A6ACCD;">)</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">value</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码使用了<code>?.</code>运算符，直接在链式调用的时候判断，左侧的对象是否为<code>null</code>或<code>undefined</code>。如果是的，就不再往下运算，而是返回<code>undefined</code>。</p><p>下面是判断对象方法是否存在，如果存在就立即执行的例子。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">iterator</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">return</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>iterator.return</code>如果有定义，就会调用该方法，否则<code>iterator.return</code>直接返回<code>undefined</code>，不再执行<code>?.</code>后面的部分。</p><p>对于那些可能没有实现的方法，这个运算符尤其有用。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#A6ACCD;"> (myForm</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">checkValidity</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">() </span><span style="color:#89DDFF;">===</span><span style="color:#A6ACCD;"> </span><span style="color:#FF9CAC;">false</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 表单校验失败</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，老式浏览器的表单对象可能没有<code>checkValidity()</code>这个方法，这时<code>?.</code>运算符就会返回<code>undefined</code>，判断语句就变成了<code>undefined === false</code>，所以就会跳过下面的代码。</p><p>链判断运算符<code>?.</code>有三种写法。</p><ul><li><code>obj?.prop</code> // 对象属性是否存在</li><li><code>obj?.[expr]</code> // 同上</li><li><code>func?.(...args)</code> // 函数或对象方法是否存在</li></ul><p>下面是<code>obj?.[expr]</code>用法的一个例子。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> hex </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">#C0FFEE</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">match</span><span style="color:#A6ACCD;">(</span><span style="color:#89DDFF;">/</span><span style="color:#C3E88D;">#</span><span style="color:#89DDFF;">([</span><span style="color:#C3E88D;">A-Z</span><span style="color:#89DDFF;">]+)/</span><span style="color:#F78C6C;">i</span><span style="color:#A6ACCD;">)</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">[</span><span style="color:#F78C6C;">1</span><span style="color:#A6ACCD;">]</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面例子中，字符串的<code>match()</code>方法，如果没有发现匹配会返回<code>null</code>，如果发现匹配会返回一个数组，<code>?.</code>运算符起到了判断作用。</p><p>下面是<code>?.</code>运算符常见形式，以及不使用该运算符时的等价形式。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">b</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于</span></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">==</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">null</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">?</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">undefined</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> a</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">b</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">[x]</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于</span></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">==</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">null</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">?</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">undefined</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> a[x]</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#82AAFF;">b</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于</span></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">==</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">null</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">?</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">undefined</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> a</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">b</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span>
<span class="line"><span style="color:#82AAFF;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于</span></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">==</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">null</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">?</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">undefined</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">a</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>上面代码中，特别注意后两种形式，如果<code>a?.b()</code>和<code>a?.()</code>。如果<code>a?.b()</code>里面的<code>a.b</code>有值，但不是函数，不可调用，那么<code>a?.b()</code>是会报错的。<code>a?.()</code>也是如此，如果<code>a</code>不是<code>null</code>或<code>undefined</code>，但也不是函数，那么<code>a?.()</code>会报错。</p><p>使用这个运算符，有几个注意点。</p><p>（1）短路机制</p><p>本质上，<code>?.</code>运算符相当于一种短路机制，只要不满足条件，就不再往下执行。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">[</span><span style="color:#89DDFF;">++</span><span style="color:#A6ACCD;">x]</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于</span></span>
<span class="line"><span style="color:#A6ACCD;">a </span><span style="color:#89DDFF;">==</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">null</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">?</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">undefined</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> a[</span><span style="color:#89DDFF;">++</span><span style="color:#A6ACCD;">x]</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，如果<code>a</code>是<code>undefined</code>或<code>null</code>，那么<code>x</code>不会进行递增运算。也就是说，链判断运算符一旦为真，右侧的表达式就不再求值。</p><p>（2）括号的影响</p><p>如果属性链有圆括号，链判断运算符对圆括号外部没有影响，只对圆括号内部有影响。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">(a</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">b)</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">c</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等价于</span></span>
<span class="line"><span style="color:#A6ACCD;">(a </span><span style="color:#89DDFF;">==</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">null</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">?</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">undefined</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> a</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">b)</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">c</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>?.</code>对圆括号外部没有影响，不管<code>a</code>对象是否存在，圆括号后面的<code>.c</code>总是会执行。</p><p>一般来说，使用<code>?.</code>运算符的场合，不应该使用圆括号。</p><p>（3）报错场合</p><p>以下写法是禁止的，会报错。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 构造函数</span></span>
<span class="line"><span style="color:#89DDFF;">new</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"><span style="color:#89DDFF;">new</span><span style="color:#A6ACCD;"> a</span><span style="color:#89DDFF;">?.</span><span style="color:#82AAFF;">b</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 链判断运算符的右侧有模板字符串</span></span>
<span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#89DDFF;">`</span><span style="color:#C3E88D;">{b}</span><span style="color:#89DDFF;">`</span></span>
<span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#82AAFF;">b</span><span style="color:#89DDFF;">`</span><span style="color:#C3E88D;">{c}</span><span style="color:#89DDFF;">`</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 链判断运算符的左侧是 super</span></span>
<span class="line"><span style="color:#A6ACCD;">super</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"><span style="color:#A6ACCD;">super</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">foo</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 链运算符用于赋值运算符左侧</span></span>
<span class="line"><span style="color:#A6ACCD;">a</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">b </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> c</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><h2 id="_3-null-判断运算符" tabindex="-1">3.Null 判断运算符 <a class="header-anchor" href="#_3-null-判断运算符" aria-hidden="true">#</a></h2><p>读取对象属性的时候，如果某个属性的值是<code>null</code>或<code>undefined</code>，有时候需要为它们指定默认值。常见做法是通过<code>||</code>运算符指定默认值。</p><div class="language-"><pre><code>const headerText = response.settings.headerText || &#39;Hello, world!&#39;;
const animationDuration = response.settings.animationDuration || 300;
const showSplashScreen = response.settings.showSplashScreen || true;
</code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面的三行代码都通过<code>||</code>运算符指定默认值，但是这样写是错的。开发者的原意是，只要属性的值为<code>null</code>或<code>undefined</code>，默认值就会生效，但是属性的值如果为空字符串或<code>false</code>或<code>0</code>，默认值也会生效。</p><p>为了避免这种情况，ES2020引入了一个新的 Null 判断运算符<code>??</code>。它的行为类似<code>||</code>，但是只有运算符左侧的值为<code>null</code>或<code>undefined</code>时，才会返回右侧的值。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> headerText </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> response</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">settings</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">headerText </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">Hello, world!</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> animationDuration </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> response</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">settings</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">animationDuration </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">300</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> showSplashScreen </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> response</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">settings</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">showSplashScreen </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> </span><span style="color:#FF9CAC;">true</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，默认值只有在左侧属性值为<code>null</code>或<code>undefined</code>时，才会生效。</p><p>这个运算符的一个目的，就是跟链判断运算符<code>?.</code>配合使用，为<code>null</code>或<code>undefined</code>的值设置默认值。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">const</span><span style="color:#A6ACCD;"> animationDuration </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> response</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">settings</span><span style="color:#89DDFF;">?.</span><span style="color:#A6ACCD;">animationDuration </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">300</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，如果<code>response.settings</code>是<code>null</code>或<code>undefined</code>，或者<code>response.settings.animationDuration</code>是<code>null</code>或<code>undefined</code>，就会返回默认值300。也就是说，这一行代码包括了两级属性的判断。</p><p>这个运算符很适合判断函数参数是否赋值。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">Component</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">props</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">const</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">enable</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">props</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">enabled</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">??</span><span style="color:#F07178;"> </span><span style="color:#FF9CAC;">true</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// …</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码判断<code>props</code>参数的<code>enabled</code>属性是否赋值，基本等同于下面的写法。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">Component</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">props</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#C792EA;">const</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    enabled</span><span style="color:#89DDFF;">:</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">enable</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#FF9CAC;">true</span><span style="color:#89DDFF;">,</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">props</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// …</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p><code>??</code>本质上是逻辑运算，它与其他两个逻辑运算符<code>&amp;&amp;</code>和<code>||</code>有一个优先级问题，它们之间的优先级到底孰高孰低。优先级的不同，往往会导致逻辑运算的结果不同。</p><p>现在的规则是，如果多个逻辑运算符一起使用，必须用括号表明优先级，否则会报错。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 报错</span></span>
<span class="line"><span style="color:#A6ACCD;">lhs </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> middle </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> rhs</span></span>
<span class="line"><span style="color:#A6ACCD;">lhs </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> middle </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> rhs</span></span>
<span class="line"><span style="color:#A6ACCD;">lhs </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> middle </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> rhs</span></span>
<span class="line"><span style="color:#A6ACCD;">lhs </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> middle </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> rhs</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面四个表达式都会报错，必须加入表明优先级的括号。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#A6ACCD;">(lhs </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> middle) </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> rhs</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">lhs </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> (middle </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> rhs)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">(lhs </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> middle) </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> rhs</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">lhs </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> (middle </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> rhs)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">(lhs </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> middle) </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> rhs</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">lhs </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> (middle </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> rhs)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">(lhs </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> middle) </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> rhs</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">lhs </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> (middle </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> rhs)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><h2 id="_4-逻辑赋值运算符" tabindex="-1">4.逻辑赋值运算符 <a class="header-anchor" href="#_4-逻辑赋值运算符" aria-hidden="true">#</a></h2><p>ES2021 引入了三个新的逻辑赋值运算符（logical assignment operators），将逻辑运算符与赋值运算符进行结合。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 或赋值运算符</span></span>
<span class="line"><span style="color:#A6ACCD;">x </span><span style="color:#89DDFF;">||=</span><span style="color:#A6ACCD;"> y</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于</span></span>
<span class="line"><span style="color:#A6ACCD;">x </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> (x </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> y)</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 与赋值运算符</span></span>
<span class="line"><span style="color:#A6ACCD;">x </span><span style="color:#89DDFF;">&amp;&amp;=</span><span style="color:#A6ACCD;"> y</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于</span></span>
<span class="line"><span style="color:#A6ACCD;">x </span><span style="color:#89DDFF;">&amp;&amp;</span><span style="color:#A6ACCD;"> (x </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> y)</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// Null 赋值运算符</span></span>
<span class="line"><span style="color:#A6ACCD;">x </span><span style="color:#89DDFF;">??=</span><span style="color:#A6ACCD;"> y</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 等同于</span></span>
<span class="line"><span style="color:#A6ACCD;">x </span><span style="color:#89DDFF;">??</span><span style="color:#A6ACCD;"> (x </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> y)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>这三个运算符<code>||=</code>、<code>&amp;&amp;=</code>、<code>??=</code>相当于先进行逻辑运算，然后根据运算结果，再视情况进行赋值运算。</p><p>它们的一个用途是，为变量或属性设置默认值。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#676E95;font-style:italic;">// 老的写法</span></span>
<span class="line"><span style="color:#A6ACCD;">user</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">id </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> user</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">id </span><span style="color:#89DDFF;">||</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 新的写法</span></span>
<span class="line"><span style="color:#A6ACCD;">user</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">id </span><span style="color:#89DDFF;">||=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面示例中，<code>user.id</code>属性如果不存在，则设为<code>1</code>，新的写法比老的写法更紧凑一些。</p><p>下面是另一个例子。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">example</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">opts</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">opts</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">foo</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">opts</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">foo</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">??</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">bar</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">opts</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">baz</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">??</span><span style="color:#F07178;"> (</span><span style="color:#A6ACCD;">opts</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">baz</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">qux</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面示例中，参数对象<code>opts</code>如果不存在属性<code>foo</code>和属性<code>baz</code>，则为这两个属性设置默认值。有了“Null 赋值运算符”以后，就可以统一写成下面这样。</p><div class="language-js line-numbers-mode"><pre><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">example</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">opts</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">opts</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">foo</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">??=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">bar</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">opts</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">baz</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">??=</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">qux</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div></div></div><!----></main><!--[--><!--]--><!----></div></div></div></div><div class="visually-hidden" aria-live="polite" data-v-23306c50>08【运算符的扩展】 has loaded</div></div></div>
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